It can be used any number of ways or not at all. You can have it switch from one to the other when the first is full, or backup files to both cards, or write RAW to one card and jpg to the other, and maybe some other options that I've forgotten.
I keep an 8GB CF and a 4GB SD in my Mark II. I can shoot almost 1500 RAW frames at a soccer match without having to open the camera in the field.
Nill
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